Real-Time Communication

Abstract

The focus of this chapter is on the architectural view of real-time communication. The chapter commences by summarizing the requirements of a real-time communication system: low protocol latency with minimal jitter, the establishment of a global time base, fast error detection at the receiver, and the need for temporal error containment by the communication system, such that a babbling node cannot hinder the communication among the correct nodes. The next section presents a waistline model of a real-time communication system. At the center of the waist is the basic message transport service (BMTS) that transports a message from a sender to a set of receivers within a given latency and with a given reliability. In real-time systems, the tradeoff between reliability and timeliness has to remain in the hands of the application and should not be hardwired in the BMTS. The protocols above the BMTS, called higher-level protocols, implement services that require the bidirectional exchange of messages such as a simple request-reply service. The protocols below the BMTS, called lower-level protocols, implement the basic message transport service. The important topic of flow control, the different types of flow control and the phenomenon of thrashing are discussed in the following section. From the temporal point of view, three different communication services can be distinguished: event-triggered communication, rate-constrained communication, and time-triggered communication. The section on event-triggered communication contains the Ethernet protocol, the CAN protocol, and the UDP protocol from the Internet suite of protocols.